My board came in today and it looks good! I can't wait to get home and try it out. I ordered two, just in case I royally mess the first one up on assembly. I really would hate to have to wait a whole extra month. I'll post more pics when I get it all put together.

Fitting into the box:http://www.flickr.com/photos/43268066@N00/3508647580/The screw holes line up perfectly. The connectors on each end were a bit taller than I had expected, though. The lid still closes, but it pinches the wires a bit. I'll trim a little useless plastic off the top of the connectors and it should be fine.

I'm not exactly an EE, so I'm real green on most of this. I knew that car voltages can be nasty, but as per other discussions around these forums, people have been running Arduinos from cars seemingly fine. The datasheet for the Nano says it should be ok up to 20v. I hadn't even considered reverse voltages at all, does this tend to happen on a car? For reverse voltage, one would throw in a diode on the power line? As for the spikes, a capacitor on the power line?

Honestly the only thing I considered was a drop in voltage on cranking the car. I doubt much that anyone is going to be using the keypad while cranking, so I didn't think a restart would hurt anything. I suppose that the dip could lock up the chip instead of just restarting it. I'll be actually running it in my door soon, so I'll have to see.

If you have any good suggestions for cleaning up the power, or maybe a good place to look, let me know.

So the 1N diode keeps the current from flowing backwards, the zener diode would cut out voltages above 15v, and the choke would clean up the power a bit. What exactly do the two different caps do? Decoupling to clean up the power? Something else?

I assume that everything to the right of C3 I should ignore, as I don't want to reduce the voltage to 5v?

Would this work for L1?http://www.mouser.com/Search/ProductDetail.aspx?qs=sGAEpiMZZMsg%252by3WlYCkUzVCd1mk%252bcol1TAFnl1vgYE%3d

And for C1, can I use a ceramic here, or only electrolytic? I'm thinking of longevity and temperature sensitivity, but I'm not terribly sure of when I can use one. If ceramics are non-polar, can they always replace the others, but not the other way around (like when dealing with AC)?

for C1 it should be a ultra low ESR electrolytic. Thats becouse they are like the "fastest" and you can get them with big F values.elextrolytic is fine, as long as you dont take the super cheap ones.my C1: http://shop.conrad.at/ce/de/product/445903/KONDENSATORULTRA-LOW-ESR220F35Vits rated with 6000h at 105°C, that should be enough...

in your chart you have a 220pF, what should be a 220µF and with a 20V zener diode your still safe but it will not be so close to trigger unwanted (14,4V is quite common in a car)